The present invention relates to rearview mirrors, and more particularly to rearview mirrors capable of automatically changing their reflective states in response to varying light levels.
High light levels reflected from a rearview mirror into the eyes of a vehicle operator during night driving can seriously impair the operator's forward vision. Light scatters within the eye causing a "veil" or "curtain" of light over the scene. The driver therefore has a reduced ability to detect objects which are dimly lit or have little contrast with the background. This situation is known as disibility glare. The driver is normally unable to detect this loss of visibility since there is no physical sensation associated therewith. At higher reflected light levels, discomfort glare occurs, resulting in an unpleasant physical sensation in the driver's eyes.
Manually actuated rearview mirrors have been developed which include "day" and "night" settings. These day/night mirrors include a mirror prism and a mechanism for changing the angular orientation of the prism. In the "day" setting, the mirror prism is angularly set to provide approximately 80 % reflectance of the rearward mirror image into the driver's eyes. In the "night" setting, the prism is angularly set to provide only approximately 4 % of the full reflectance of the rearward image into the driver's eyes.
Additionally, "automatic" day/night rearview mirrors have been developed which automatically switch the mirror prism between full and partial reflectance states in response to sensed light levels. One such mirror is illustrated in U.S. Pat. No.4,443,057, issued Apr. 17, 1984, to Bauer et al, entitled AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVE VEHICLES. The Bauer mirror includes a forward light sensor for measuring light in the forward direction, a rear light sensor for measuring light in the rear direction, and a control circuit responsive to the forward and rear light sensors to control the mirror prism. Additional disclosures of automatic day/night rearview mirrors are listed in the Disclosure Statement filed simultaneously herewith.
Known automatic day/night rearview mirrors all suffer a common drawback. Specifically, the light sensors (e.g. photocells, photoresistors, and photodiodes) used in these mirrors have a spectral response to the electromagnetic spectrum which is substantially different from the spectral response of the human eye. Accordingly, the sensitivities of the light sensors to visible and invisible wavelengths are different from the sensitivity of the human eye. Most notably, light sensors are extremely sensitive to infrared and longer wavelengths which cannot be seen by the human eye. Infrared wavelengths are prevalent in artificial lighting and particularly in tungsten filament bulbs. The light sensors detect relatively high light intensities when viewing headlights, taillights, street lights, or any other source of infrared wavelengths. Consequently, the reflective element of a mirror incorporating such sensors is driven to an inappropriate reflective state. The mirror therefore is actuated when not necessary to meet the sensitivity of the human eye and not actuated when necessary to meet the sensitivity of the human eye. Either inadequate image information is presented to the driver and/or excessive glare from the rearview mirror is directed into the driver's eyes.